'\" et
.TH ASCTIME "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
asctime,
asctime_r
\(em convert date and time to a string
.SH SYNOPSIS
.LP
.nf
#include <time.h>
.P
char *asctime(const struct tm *\fItimeptr\fP);
char *asctime_r(const struct tm *restrict \fItm\fP, char *restrict \fIbuf\fP);
.fi
.SH DESCRIPTION
For
\fIasctime\fR():
The functionality described on this reference page is aligned with the
ISO\ C standard. Any conflict between the requirements described here and the
ISO\ C standard is unintentional. This volume of POSIX.1\(hy2017 defers to the ISO\ C standard.
.P
The
\fIasctime\fR()
function shall convert the broken-down time in the structure pointed
to by
.IR timeptr
into a string in the form:
.sp
.RS 4
.nf

Sun Sep 16 01:03:52 1973\en\e0
.fi
.P
.RE
.P
using the equivalent of the following algorithm:
.sp
.RS 4
.nf

char *asctime(const struct tm *timeptr)
{
    static char wday_name[7][3] = {
        "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"
    };
    static char mon_name[12][3] = {
        "Jan", "Feb", "Mar", "Apr", "May", "Jun",
        "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"
    };
    static char result[26];
.P
    sprintf(result, "%.3s %.3s%3d %.2d:%.2d:%.2d %d\en",
        wday_name[timeptr->tm_wday],
        mon_name[timeptr->tm_mon],
        timeptr->tm_mday, timeptr->tm_hour,
        timeptr->tm_min, timeptr->tm_sec,
        1900 + timeptr->tm_year);
    return result;
}
.fi
.P
.RE
.P
However, the behavior is undefined if \fItimeptr\fR\->\fItm_wday\fR or
\fItimeptr\fR\->\fItm_mon\fR are not within the normal ranges as
defined in
.IR <time.h> ,
or if \fItimeptr\fR\->\fItm_year\fR exceeds
{INT_MAX}\-1990,
or if the above algorithm would attempt to generate more than 26 bytes
of output (including the terminating null).
.P
The
.BR tm
structure is defined in the
.IR <time.h> 
header.
.P
The
\fIasctime\fR(),
\fIctime\fR(),
\fIgmtime\fR(),
and
\fIlocaltime\fR()
functions shall return values in one of two static objects: a
broken-down time structure and an array of type
.BR char .
Execution of any of the functions may overwrite the information
returned in either of these objects by any of the other functions.
.P
The
\fIasctime\fR()
function need not be thread-safe.
.P
The
\fIasctime_r\fR()
function shall convert the broken-down time in the structure pointed to
by
.IR tm
into a string (of the same form as that returned by
\fIasctime\fR(),
and with the same undefined behavior when input or output is out of
range) that is placed in the user-supplied buffer pointed to by
.IR buf
(which shall contain at least 26 bytes) and then return
.IR buf .
.SH "RETURN VALUE"
Upon successful completion,
\fIasctime\fR()
shall return a pointer to the string.
If the function is unsuccessful, it shall return NULL.
.P
Upon successful completion,
\fIasctime_r\fR()
shall return a pointer to a character string containing the date and
time. This string is pointed to by the argument
.IR buf .
If the function is unsuccessful, it shall return NULL.
.SH ERRORS
No errors are defined.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
None.
.SH "APPLICATION USAGE"
These functions are included only for compatibility with older
implementations. They have undefined behavior if the resulting string
would be too long, so the use of these functions should be
discouraged. On implementations that do not detect output string
length overflow, it is possible to overflow the output buffers in such
a way as to cause applications to fail, or possible system security
violations. Also, these functions do not support localized date and
time formats. To avoid these problems, applications should use
\fIstrftime\fR()
to generate strings from broken-down times.
.P
Values for the broken-down time structure can be obtained by calling
\fIgmtime\fR()
or
\fIlocaltime\fR().
.P
The
\fIasctime_r\fR()
function is thread-safe and shall return values in a user-supplied
buffer instead of possibly using a static data area that may be
overwritten by each call.
.SH RATIONALE
The standard developers decided to mark the
\fIasctime\fR()
and
\fIasctime_r\fR()
functions obsolescent even though
\fIasctime\fR()
is in the ISO\ C standard due to the possibility of buffer overflow. The ISO\ C standard
also provides the
\fIstrftime\fR()
function which can be used to avoid these problems.
.SH "FUTURE DIRECTIONS"
These functions may be removed in a future version.
.SH "SEE ALSO"
.IR "\fIclock\fR\^(\|)",
.IR "\fIctime\fR\^(\|)",
.IR "\fIdifftime\fR\^(\|)",
.IR "\fIgmtime\fR\^(\|)",
.IR "\fIlocaltime\fR\^(\|)",
.IR "\fImktime\fR\^(\|)",
.IR "\fIstrftime\fR\^(\|)",
.IR "\fIstrptime\fR\^(\|)",
.IR "\fItime\fR\^(\|)",
.IR "\fIutime\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<time.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
